Emergent flat-band physics in $d^{9-\delta}$ multilayer nickelates

TL;DR

This paper investigates the electronic structure of layered nickelates, revealing emergent flat-band physics and complex orbital interactions that may underpin unconventional superconductivity.

Contribution

It provides a detailed many-body analysis of multilayer nickelates, highlighting the role of Ni-$d_{z^2}$ orbitals and flat bands in their low-energy physics, which is novel compared to prior studies.

Findings

Ni-$d_{x^2-y^2}$ orbitals are highly correlated near Mott-insulating regime

Emergence of non-dispersive Ni-$d_{z^2}$ flat bands at low temperature

Distinct fermiology in Nd compound dominated by Ni-$d_{z^2}$

Abstract

Recent experiments have shown that the reduced multilayer rare-earth (RE) nickel oxides of form RE$_{p+1}$Ni$_p$O$_{2p+2}$ may belong to the novel family of superconducting lanthanide nickelates. Here, the correlated electronic structure of Pr$_{4}$Ni$_3$O$_{8}$ and Nd$_{6}$Ni$_5$O$_{12}$ is studied by means of an advanced realistic many-body framework. It is revealed that the low-energy physics of both systems is dominated by an interplay of Ni-$d_{x^2-y^2}$ and Ni-$d_{z^2}$ degrees of freedom. Whilst the Ni-$d_{x^2-y^2}$ orbitals are always highly correlated near an (orbital-selective) Mott-insulating regime, the Ni-$d_{z^2}$ orbitals give rise to intriguing non-dispersive features. At low temperature, the Pr compound still displays QP-like Ni-$d_{x^2-y^2}$-derived states at the Fermi level, but the interacting fermiology of the Nd compound is outshined by an emergent Ni-$d_{z^2}$-controlling flat band. These findings translate well to the previous characterization of doped infinite-layer nickelates, and hence further make the case for a mechanism of unconventional superconductivity which is distinct from the one in high-$T_{\rm c}$ cuprates.